c++/reflection: consteval-only type in splice-spec [PR125900]

In this test in the template for the

  std::println("{}", static_cast<int>(s.[:member:]));

line provokes a bogus "consteval-only expressions are only allowed
in a constant-evaluated context" error.  (Not the line before
because there we have an OVERLOAD because the second argument's
type isn't obvious, and so the whole thing is considered type-dep.)

A splice-specifier is [: constant-expression :], and [expr.const.defns]:
an expression or conversion is manifestly constant-evaluated if it is
a constant-expression.

So I think we shouldn't walk SPLICE_EXPRs in
check_out_of_consteval_use_r.

	PR c++/125900

gcc/cp/ChangeLog:

	* reflect.cc (check_out_of_consteval_use_r): Don't walk
	SPLICE_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/reflect/expr18.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 4e56ca07b9eeb9dd765893b34c147ca398a57f71)